ABSTRACT

Wehavealreadyseenthewayinwhichairborneblastcandamagestructures.Averticalwallinthepathofablastwavewillexperiencealoading derivedfromboththestaticoverpressureandthedynamicoverpressuredue totheconcertedmovementoftheairmoleculesimpingingonthestructure. Thesepressureswillcausethewalltobendanddeflectanddamagetothe structurewillresultifthestressesorstrainswhichensueexceedthosewhich thematerialcantolerate.Atthesametimeanassociatedphenomenonis takingplace.Thestresslevelonthefrontfaceofthewallrisesandfalls suddenlyduetothearrivaloftheairbornepressurewave.Thistransient stressispropagatedthroughthewallintheformofacompressivestress wave.Onarrivalattherearfaceofthewallitisreflectedbackasatensile wave.ThisprocessisdescribedschematicallyinFigure6.1,withcompressivestressesbeingtakenaspositiveandtensilestressesasnegative.During thereflectionprocess,theleadingedgeofthetensilereflectedwaveoverlaps thetrailingedgeoftheincomingcompressivewave.Theresultantstress experiencedbythematerial,ataspecificlocation,isthereforethealgebraic sumofthetwowavesatthatpointatthatinstantintime.Theintensityofthe

Figure6.1Stresswavereflectionatafreesurface

Stress waves 1 OS

nett tensile stress therefore builds as the tensile wavefront proceeds back towards the face of the target until it achieves its full value when it passes the tail of the incoming compressive wave. Nearly all materials are weaker in tension than compression and thus a wave, which has passed uneventfully through the target, may well tear a scab from the rear face. Experience shows us that for an airborne blast wave from a source remote from the wall, scabbing is unlikely to occur. However, if the source of the explosion is near to, or in contact with, the slab, scabbing becomes the more likely form of failure. Scabbing can also occur in both structures and armoured fighting vehicles as a result of remotely delivered explosive charges, known as high explosive squash head rounds (HESH), and indeed from ballistic impact by inert projectiles.